Create TimelineEvent model

Create `IncidentManagement::TimelineEvent` model
with associations and validation rules

ee/app/models/incident_management/timeline_event.rb

+# frozen_string_literal: true
+
+module IncidentManagement
+  class TimelineEvent < ApplicationRecord
+    self.table_name = 'incident_management_timeline_events'
+
+    belongs_to :project
+    belongs_to :author, class_name: 'User', foreign_key: :author_id
+    belongs_to :incident, class_name: 'Issue', foreign_key: :issue_id
+    belongs_to :updated_by_user, class_name: 'User', foreign_key: :updated_by_user_id
+    belongs_to :promoted_from_note, class_name: 'Note', foreign_key: :promoted_from_note_id
+
+    validates :project, :incident, :occurred_at, presence: true
+    validates :action, presence: true, length: { maximum: 128 }
+    validates :note, :note_html, presence: true, length: { maximum: 10_000 }
+  end
+end

ee/spec/factories/incident_management/timeline_events.rb

+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :incident_management_timeline_event, class: 'IncidentManagement::TimelineEvent' do
+    association :project
+    association :author, factory: :user
+    association :incident
+    association :promoted_from_note, factory: :note
+    occurred_at { Time.current }
+    note { 'timeline created' }
+    note_html { '<strong>timeline created</strong>' }
+    action { 'comment' }
+  end
+end

ee/spec/models/incident_management/timeline_event_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe IncidentManagement::TimelineEvent do
+  let_it_be(:timeline_event) { create(:incident_management_timeline_event) }
+
+  describe 'associations' do
+    it { is_expected.to belong_to(:project) }
+    it { is_expected.to belong_to(:author) }
+    it { is_expected.to belong_to(:incident) }
+    it { is_expected.to belong_to(:updated_by_user) }
+    it { is_expected.to belong_to(:promoted_from_note) }
+  end
+
+  describe 'validations' do
+    subject { build(:incident_management_timeline_event) }
+
+    it { is_expected.to validate_presence_of(:project) }
+    it { is_expected.to validate_presence_of(:incident) }
+    it { is_expected.to validate_presence_of(:note) }
+    it { is_expected.to validate_length_of(:note).is_at_most(10_000) }
+    it { is_expected.to validate_presence_of(:note_html) }
+    it { is_expected.to validate_length_of(:note_html).is_at_most(10_000) }
+    it { is_expected.to validate_presence_of(:occurred_at) }
+    it { is_expected.to validate_presence_of(:action) }
+    it { is_expected.to validate_length_of(:action).is_at_most(128) }
+  end
+end